Initially, Vertex said that two of its drugs, Kalydeco and VX-809, used in combination, boosted lung function by 5% in 46% of patients, and 10% in 30% of patients in the trial.

However, yesterday, Vertex officials said that the earlier results contained “misinterpretation” of statistical data between itself and an outside statistical analysis firm that led to an overstatement of the lung function improvements for patients in the trial.

After correcting for the error, the trial showed lung function improvements of 5% in 35% of patients, and 10% in 19% of patients.

The company noted that the corrected results still showed good lung function improvement for patients taking the drugs.

Kalydeco has already been approved for certain forms of the disease, affecting about 4% of all cystic fibrosis sufferers. VX-809 is a newly developed and still experimental medication. The combination of the drugs was aimed at treating about half of all people with the disease.